The following post is mainly conjecture:
From what I can tell, the Master/Slave melzi configuration works by connecting the TX, RX, SCL, and Reset lines between the two boards and communicating over custom firmware. It shouldn't be too hard to do the same thing with two Arduino's, although I'm not sure what the SCL line is for (my understanding is that SCL is for I2C). From there, its just a matter of firmware. I'm not sure if the RepRapPro Marlin will run on a RAMPS board.